The present invention relates to an adsorption process and an apparatus for the production of nitrogen; in particular the invention relates to a process and an apparatus for the production of gaseous nitrogen at preset and constant purity under conditions of variable pressure and flowrate of the gas supplied to the final user. The invention also relates to the use of said apparatus in drink dispensers, such as beer and wine dispensers.
In many nitrogen gas applications, such as the one cited above or for example in plants for food packaging in protected or modified atmosphere, if on the one side it is desirable to avoid using cylinders connected to the apparatus that uses the gaseous nitrogen, on the other consumptions or room available are such as to make impossible the use of large fixed reservoirs of the type used in hospitals and research laboratories. Rather, for above applications in situ production devices are used, that are based on oxygen adsorption from a compressed air flow that is passed through columns packed with carbon (or coke) molecular sieves; the exiting nitrogen is fed to a collecting reservoir and hence to the final use, e.g. a beer tapping column or dispenser.
U.S. Pat. No. 4,572,723 describes an apparatus comprising a compressor, means for filtering and dehumidifying compressed air, an adsorber vessel containing molecular sieves and a reservoir to receive the purified nitrogen exiting the adsorber vessel. The regeneration of the molecular sieves is achieved by depressurising the adsorber vessel: by this operation almost all adsorbed oxygen is vented to the atmosphere.
One of the problems of the known devices is that nitrogen purity is not fixed and constant being a variable depending on some factors such as: the rate and flow of the compressed air stream, the amount of nitrogen produced in each cycle, the pressure of nitrogen collected in the reservoir, which, on its turn, is a variable depending on the flow of nitrogen drawn and sent to the final user. To overcome this drawback some solutions have been proposed: one of these envisages the introduction of a buffer tank to be placed between the adsorber column and the collecting reservoir; another is that of using two adsorbers (e.g. in U.S. Pat. No. 4,925,461 and in U.S. Pat. No. 5,382,280) and to have the exiting flow move from the first to the second adsorber to remove oxygen traces. These solutions however involve cost increases, greater room needs and longer production times; moreover, they do not solve the problem in that they do not ensure the required fixed and constant purity of gaseous nitrogen product.
The object of the present invention is to solve the aforesaid problems and to provide an adsorption process and apparatus for the production of nitrogen gas that has low costs and results in nitrogen generation at the desired purity (generally with oxygen content within the range of 5 ppm to 2%) without being strictly bound to the flowrate value of the feed air stream.
A further object of the invention is to provide an apparatus capable of transferring from the molecular sieve vessel to the collecting reservoir, over the time of a cycle duration, a calculated and established amount of nitrogen gas, at a predetermined transfer pressure, independently of the pressure value in the collecting reservoir, of the instant nitrogen consumption and, within certain limits, of the flowrate of the feed air without being strictly bound to the pressure value of the reservoir that can vary in the range of 1 to 10 bar and without limitations to the gas instant drawing that can exceed the specified values.
These objects are achieved by means of the present invention; that relates to a process for the production of nitrogen comprising the steps of: feeding compressed air to an adsorber cylinder containing a plurality of adsorbent means for removing oxygen from said air until said adsorber cylinder is pressurized; feeding product gas flow exiting said cylinder to a collecting reservoir; depressurising said adsorber cylinder to regenerate said adsorbent means and cyclically repeating said steps, characterized in further comprising the steps of: calculating the amount Q1 of nitrogen gas obtainable from said adsorber cylinder with the required purity at pressure Pp of the cylinder; bringing said cylinder to a pressure value at least equal to said pressure Pp and in any case exceeding the pressure present in said collecting reservoir; and transferring in each cycle from said adsorber cylinder to said reservoir a fixed gas amount Q2, where Q1xe2x89xa7Q2.
The process includes three steps:
Pressurisation step during which air is compressed within the cylinder until at least the preset pressure value Pp or in any case a value exceeding Pp and capable of transferring nitrogen from the cylinder 2 to the tank 3 is reached.
Transfer step during which the amount Q2 of nitrogen gas at the desired purity is transferred from the cylinder into the collecting reservoir: the transferred amount is constant and controlled by a flow regulator.
Depressurisation step during which oxygen is vented from the cylinder.
According to a feature of the invention pressure is measured inside the adsorber cylinder and inside the collecting reservoir and if the cylinder pressure is lower than the tank pressure, before starting the transfer step compressed air continues to be fed to the cylinder until a pressure value exceeding that of the tank is reached.
According to a preferred feature of the invention the transfer step is carried out at preset and constant flowrate and the flow of the transferred gas is shut off after a present and constant time for each cycle. According to a further feature of the invention, compressed air is fed to said cylinder during said transfer step and compressed air feeding is interrupted at the end of the transfer step.
The invention also relates to an apparatus for nitrogen production comprising a compressed air source (1, 1A); at least one cylinder (2) containing means for oxygen adsorption; a reservoir (3) to collect the product gas exiting said cylinder and means (7) to transfer said gas from said cylinder (2) to said reservoir (3), wherein said apparatus further comprises means (19, 20) to measure pressure in said cylinder (2) and in said reservoir (3); means (9) to measure and regulate said gas flow in said transfer means, said regulating means (9) being set to transfer a fixed and constant volume of gas; and means (8) to stop said transfer once a preset amount Q2 of gas has been transferred.
According to a preferred embodiment of the invention the apparatus also includes means to measure and regulate the duration of the transfer time, the pressurisation time and the depressurisation or venting time.
Preferably, the measuring and regulating means include at least a pressure gauge, a timer, a flow regulator and an on-off valve, all connected to a microprocessor controlled system.
According to a further feature of the invention, the apparatus also includes means to cut off the compressed air stream feeding or, if provided, the compressor operation, both once the required nitrogen amount Q2 is transferred or in case a maximum preset pressure in the reservoir should be reached due to a reduced gas consumption by the user.
As mentioned above, a particularly advantageous application of the apparatus according to the invention is in apparatuses for dispensing or drawing drinks such as beer, wine and beverages in general. It is therefore a further object of the invention the use of the aforedescribed apparatus, as defined in claim 10. Another object of the invention is therefore a device for delivering Pr drawing drinks comprising an apparatus of the aforedescribed type.
As mentioned above, the apparatus and process of the invention allow to obtain several advantages versus the prior art.
First of all the transfer of nitrogen from the cylinder to the reservoir is totally independent of the pressure of the gas collected inside said reservoir. In fact, according to the invention, a preset gas amount is transferred from the cylinder to the reservoir independently of the pressure in the reservoir, in order to avoid that the transferred nitrogen has a purity degree other than the required one. Another advantage is that, unlike other known processes where a supplementary reservoir is introduced, named Buffer Tank or Surge Tank, which must necessarily be maintained at a reference pressure, the process of the invention allows to eliminate the Buffer Tank and the pressure in the collecting reservoir may vary in the range of 1 to 10 bar without involving any problem for the purity and generation of nitrogen. Thanks to this feature it is possible to draw more nitrogen gas from the reservoir than it was possible with the traditional systems, without the transferred nitrogen being polluted by a higher oxygen percentage than the established one.
Moreover, the systems for dispensing and drawing drinks including the apparatus according to the invention are more flexible and easy to be used than the traditional ones: for example, the apparatus ensures the interchangeability of the compressor, if necessary, without requiring adjustments and/or calibrations in case the compressor is replaced with another compressor having different characteristics within a 20% range; this result is not achievable in traditional systems.